Re: [fuse-devel] [PATCH] FUSE/CUSE: implement direct mmap support

[Goswin von Brederlow <goswin-v-b@web.de>]

Tejun Heo <tj@kernel.org> writes:

> Implement FUSE direct mmap support.  The server can redirect client
> mmap requests to any SHMLBA aligned offset in the custom address space
> attached to the fuse channel.  The address space is managed by the
> server using mmap/munmap(2).  The SHMLBA alignment requirement is
> necessary to avoid cache aliasing issues on archs with virtually
> indexed caches as FUSE direct mmaps are basically shared memory
> between clients and the server.
>
> The direct mmap address space is backed by pinned kernel pages which
> are allocated on the first fault either from a client or the server.
> If used carelessly, this can easily waste and drain memory.
> Currently, a server must have CAP_SYS_ADMIN to manage dmmap regions by
> mmapping and munmapping the channel fd.

Does that mean that for example in unionfs-fuse when a user wants to
mmap a file I can just mmap the actual underlying file from the real
filesystem and any read/write access would then shortcut fuse and go
directly to the real file?

MfG
        Goswin